Density is mass per unit volume of a substance. (g/cc).

Lecture Three

Density
Density is mass per unit volume of a substance. (g/cc).
Thus, if 10 mL of olive oil weighs 9.1 g, its density is:


Specific Gravity
Specific gravity (sp gr) is a ratio, expressed decimally, of the weight of a substance to the weight of an equal volume of a substance chosen as a standard (Water for liquids and solids; while hydrogen for gases).

Thus, if 10 mL of olive oil weighs 9.1 g, and 10 mL of water, under similar conditions, weighs 10 g, the specific gravity of the oil is:

• Substances that have a specific gravity less than 1 are lighter than water.
• Substances that have a specific gravity greater than 1 are heavier than water.
In the United States Pharmacopeia, the standard temperature for specific gravities is 25°C, except for that of alcohol, which is 15.56°C by government regulation.



Density Versus Specific Gravity
The density has various units of measure, while specific gravity has no dimension and is therefore a constant value for each substance (when measured under controlled conditions). Thus, whereas the density of water may be variously expressed as 1 g/mL, 1000 g/L, or 62.5 lb/cu ft, the specific gravity of water is always 1.


Table 5.1 presents some representative specific gravities. Figure 1 depicts the layering of immiscible liquids due to their relative weights.



Calculating the Specific Gravity of Liquids Known Weight and Volume

Calculating the specific gravity of a liquid when its weight and volume are known involves the use of the equation:

Examples: page 69

If 54.96 mL of an oil weighs 52.78 g, what is the specific gravity of the oil?
54.96 mL of water weighs 54.96 g

Pycnometer or Specific Gravity Bottle
A pycnometer is a special glass bottle used to determine specific gravity (Fig. 2). Pycnometers are generally available for laboratory use in volumes ranging from 1 mL to 50 mL. Pycnometers have fitted glass stoppers with a capillary opening to allow trapped air and excess fluid to escape.

In using a pycnometer, it is first weighed empty and then weighed again when filled to capacity with water. The weight of the water is calculated by difference. Since 1 g of water equals 1 mL, the exact volume of the pycnometer becomes known. Then, when any other liquid subsequently is placed in the pycnometer, it is of equal volume to the water, and its specific gravity may be determined.


Example:
A 50 mL pycnometer is found to weigh 120 g when empty, 171 g when filled with water, and 160 g when filled with an unknown liquid. Calculate the specific gravity of the unknown liquid.